In a magnetic recording/reproducing apparatus such as a videocassette recorder, as a head deviates from a track on a magnetic recording medium during playback, head output is decreased and errors increase. This precludes the normal reproduction of an image, so it is required for the head to trace a target track precisely. In other words, it is necessary to maintain head tracking. In order to extend recording time in a digital videocassette recorder for home use, tracks are especially narrow, which increases the precision of the head tracking needed for satisfactory reproduction of images. Among the methods for detecting head tracking error, or deviation from ideal tracking, are methods that use different respective pilot signals for successive tracks to facilitate comparison of the crosstalk of the pilot signals from the tracks preceding and succeeding the track being most closely followed by the head, thus to detect whether the head tracking deviates toward the preceding track or toward the succeeding track. The pilot signals take the form of peaks and notches in the frequency spectra of the digital signals recorded on the tracks by selectively recording one of two types of interleaved non-return to-zero, invert-on-ONEs (I-NRZI) modulation. The same information is precoded into two parallel-in-time sets of serially supplied channel words; and the channel words that are selected from one or the other of the sets to control I-NRZI modulation during recording, are selected so the I-NRZI modulation will deviate least from the pilot signal criterion for each recording track. When the selection of the channel word is completed, precoding information stored in the precoder that did not generate the selected channel word is altered, to conform to precoding information stored in the precoder that did generate the selected channel word. This is done to provide continuity of the precoding procedures and of the decoding procedures subsequent to the I-NRZI modulation being recovered from the recording medium during playback and demodulated. When the selection of the channel word is completed, integrators in the circuitry for determining which channel word is to be selected have to have their contents updated to reflect which channel word was in fact selected for recording. Such methods are described in U.S. Pat. No. 5,142,421 issued 25 Aug. 1992 to Kahlman et alii, entitled "DEVICE FOR RECORDING A DIGITAL INFORMATION SIGNAL ON A RECORD CARRIER" and incorporated herein by reference.
In Kahlman et alii the generation of the I-NRZI modulation is done on a serial-bit basis. This does not lend itself to pipeline operation in which channel words selected from the serial-bit precoders are recorded on the magnetic recording medium, after some fixed delay to accommodate the selection circuitry. It takes some time after a pair of respective channel words are generated, for a decision procedure that determines which of them will be recorded. After the decision procedure, it then takes some further time for updating stored information in the precoders. After the decision procedure, it then takes some further time for updating stored information in the precoders. These decision and updating procedures must be completed before further precoding is possible, so the delays caused by these decision and updating procedures introduce gaps into the continuous flow of bits as regularly clocked by synchronous clocking methods. Accordingly, first-in/first-out buffer storage that can be intermittently read from has to be provided before the serial-bit precoders; and first-in/first-out buffer storage that can be intermittently written with the selected channel words and subsequently continuously read from has to be provided for channel words generated by the serial-bit precoders. The generation of clocking signals for the buffer storage is somewhat complex, so it is desired to avoid the need for intermittently written or intermittently read buffer storage.
A application entitled DIGITAL SIGNAL RECORDING APPARATUS filed 7 Jun. 1995 by the inventor is included herein by reference, as non-essential subject matter. In this previously filed application the inventor, Soon-Tae Kim, describes using parallel-bit precoders together with parallel-bit-to-serial-bit (P/S) converters, to implement pipeline processing of the generation of I-NRZI modulation and to avoid the need for intermittently written or intermittently read buffer storage. The precoding procedures in the initial steps of the generation of I-NRZI modulation are generally carried out using first and second precoders of 2T type, to generate in parallel the two sets of channel words from which the channel words are selected for recording. As Kahlman et alii point out, when single-bit prefixes are affixed to the information words precoded by first and second precoders of 2T type, the corresponding odd bit-places in the respective channel words they concurrently generate are bit complements of each other, and the corresponding even bit-places in these channel words are the same. This property is used to reduce the amount of parallel-bit-to-serial-bit conversion required after first and second parallel-bit precoders of 2T type, in certain of the digital signal recording apparatus described in the inventor's above-referenced concurrent patent application.
In embodiments of the inventions described herein, this property is exploited for generating I-NRZI modulation, using a lone precoder of 2T type to generate the first of a pair of concurrent channel words and generating the second channel word of the pair from the first by bit-complementing its odd bit-places and retaining its even bit-places unchanged. It is particularly attractive to use a lone precoder of 2T type when the precoding is done on a parallel-bit basis, since there are a considerable number of exclusive-OR gates and a considerable number of bit latches in a parallel-bit precoder of 2T type, as described in the inventor's above-referenced concurrent patent application.